Long Term Evolution (LTE) is a wireless communication network technology standardized by the 3rd Generation Partnership Project (3GPP). LTE communication networks support a packet-switched domain rather than a circuit-switched domain as in previous 3GPP systems. Thus, data may be moved in a LTE network using separate small blocks of data (e.g., packets). The LTE standard is based on orthogonal frequency-division multiplexing (OFDM) in the downlink, and single carrier frequency division multiple access (SC-FDMA) in the uplink. In the time domain in the downlink, one subframe is divided into a number of OFDM symbols. Thus, one OFDM symbol may consist of a number of sub-carriers in the frequency domain.
In LTE, no dedicated channels are used for the transmission of user data. Instead, shared channel resources are used in both the downlink and uplink. Shared resources are controlled by a scheduler that assigns different parts of the downlink and uplink shared channels to different wireless communication devices (WCDs) (e.g., smartphones, tablets, phablets, personal computers, etc.) for reception and transmission, respectively.
Release 8 of the 3GPP standard introduced cell-specific reference signals (CRS) in the downlink designed for use in channel estimation for up to 4-layer spatial multiplexing, with separate CRS sequences for each antenna port. However, with the addition of up to 8-layer spatial multiplexing in Release 10 of the 3GPP standard came the need for 8-layer channel estimation.
Because extending CRS to 8 layers would add more signaling overhead than otherwise desired, Release 10 introduces the Channel State Information Reference Signal (CSI-RS). CSI-RS is transmitted from a base station on different antenna ports than CRS such that CRS measurements are not interfered by CSI-RS transmissions. Additionally, instead of using only time/frequency orthogonally as in CRS, CSI-RS further uses code-domain orthogonality. CSI-RS may be regularly transmitted by a base station and may be used for measurement purposes and estimating channel state quality by WCDs compliant with Release 10.
Link Adaptation (LA) is an important function in a wireless communication network. In LTE, the purpose of LA is to select a modulation and coding scheme (MCS) to use when transmitting data (e.g., a transport block) to a WCD. Typically, an MCS is selected for each transport block, which is transmitted every transmission time interval (TTI).
A conventional LA process selects an appropriate MCS based on a channel quality value (e.g., a Signal to Interference plus Noise Ratio (SINR) estimate) and an offset value (offset) (this offset value is sometimes referred to as the “outer-loop” offset value). For example, a conventional LA process calculates an adjusted SINR (SINRa) based on the estimated SINR (SINRe) and the outer-loop offset value (e.g., SINRa=SINRe+offset) and then uses SINRa to pick an MCS (e.g., an MCS index) from a set of available MCSs. For example, once the adjusted SINR (SINRa) is calculated, the base station can use a look-up table to select the MCS that corresponds to SINRa.
The channel quality value (e.g., SINR) may be determined based on channel state information (CSI) reported to the base station by the WCD for which the MCS is being selected. The outer-loop offset value is dependent on a Hybrid Automatic Repeat Request (HARQ) message (i.e., HARQ ACK or HARQ NACK) transmitted by the WCD and received by the base station. A conventional LA process calculates the offset as follows: offset=offset+ACKoffset or offset=offset+NACKoffset, depending on whether a HARQ ACK or HARQ NACK was received, respectively.